When taken immediately after surgery for five years, tamoxifen cut the recurrence rate over the next 10 years in half regardless of age. It also reduced the risk of developing a new cancer in the other breast, according to the study, described as the largest cancer study ever undertaken.
"The evidence is getting to be quite strong that these are permanent preventions, in other words, a cure," said Dr. Harmon Eyre, chief medical officer of the American Cancer Society. "This takes it beyond any question of scientific debate."
The study was headed by scientists at Oxford University and published in this week's issue of The Lancet, a British medical journal. It is part of an ongoing analysis of all tamoxifen studies ever conducted worldwide and followed 37,000 women with early breast cancer.
In most cases, breast cancer spreads as a result of hormones that encourage it to grow. Tamoxifen attacks spreading cancer by blocking these hormones. The drug does not help when women's tumors are insensitive to hormones.
With one million women worldwide taking it, tamoxifen is one of the most widely used cancer drugs. But it is most often given to post-menopausal women. Younger women are mostly treated with chemotherapy alone, because doctors did not believe tamoxifen helped. They theorized that high hormone levels in pre-menopausal women might overwhelm the drug.
But the new findings showed tamoxifen was effective for women of all ages with hormone-sensitive cancer, whether or not chemotherapy had been given and whether or not the cancer had spread to their lymph nodes.
"Starting five years of tamoxifen immediately after surgery prevented one in six women from relapsing and one in 12 from dying," said the study's leader, Richard Peto, professor of medical statistics and epidemiology at Oxford.
"Tamoxifen for older women is already saving more lives than any other cancer drug, but most of the young breast cancer patients who need tamoxifen are not getting it."
Early findings in the mid-1980s showed tamoxifen could improve five-year survival. By 1992, the drug was shown to improve 10-year survival as well, but that was based on prescribing it for two years or less.
The latest information shows, however, that five years of tamoxifen is about twice as effective.
Part of the reason why tamoxifen is not being prescribed as widely as it could be is because the drug is linked to cancer of the womb and blood clots in the lung.
But overall, for women who have already had breast cancer the drug prevents about 30 times as many deaths as it causes, the study found.
Tamoxifen is also sometimes given to healthy women at high risk of developing breas cancer as a way of preventing the disease, but researchers warn that for these women the balance between tamoxifen's benefits and adverse effects is uncertain.
By Emma Ross